A few years ago someone had reported that their Saito engine blew off its cylinder head in a similar manner. But I don't remember the details now. It might be way back in the old history files someplace too.
Someone else mentioned that it was possible to still have a hydraulic lock occur, at or near TDC. In that case it may not damage the piston or rod. There just might be the right conditions to get the fuel to ignite in that situation too. Or the cylinder head came off and as it was getting pushed off the air fuel mixture ignited causing the bang.